1. Field of the Invention
This invention relates to a method of transforming colors of image such that the appearances of perceived colors may become identical between images, which are formed with devices, such as a color printer and a color display device, that have different correlated color temperatures of white points, different luminance levels, and the like, and which are viewed at different brightness levels of the surround.
2. Description of the Related Art
Images have heretofore been formed independently by various media, such as printing devices, photographic devices, and televisions, and used. However, with the rapid advances made in the information industry in recent years, there has arisen a tendency toward increased use of the so-called "media mix" techniques, with which the images are formed by transforming the image information between different media. Under such circumstances, the image read-out, processing, and reproducing methods which are referred to as the "digital imaging techniques" became popular. Specifically, with the digital imaging techniques, the image information having been reproduced on a photograph is displayed on various kinds of display devices utilizing emitted light or transmitted light (various kinds of display systems utilizing phosphors, liquid crystals, plasmas, or the like), and the displayed image information is used. Also, for example, character patterns are added to the image having been reproduced on the display device, or a portion of the displayed image is altered or corrected. Further, an image having been formed on a display device is outputted to and reproduced by various kinds of printers (such as ink jet printers, sublimation types of printers, hot-melt types of printers, and printers utilizing silver halides), and the reproduced image is stored.
With the digital imaging techniques, images on hard copies, such as prints and photographs, are viewed as the images (hereinbelow referred to as the soft copies) formed on various different display devices. Therefore, as for a color image, the color appearances vary in accordance with the kind of the image recording or displaying medium. Accordingly, there is a strong demand for a technique for transforming the colors of an image such that the same color appearances can be obtained on every medium.
Specifically, ordinarily, a display image (i.e., a soft copy image) displayed on a CRT display device, or the like, and a hard copy image formed by a printer, or the like, differ markedly in the tint of the viewing illuminant, the brightness of the reference white, and the ambient conditions under which the image is viewed. For example, the white color of a CRT display device ordinarily has a correlated color temperature falling within the range of 6500K to 9300K, and an ordinary domestic fluorescent light has a correlated color temperature falling within the range of 4000K to 5000K. Also, hard copies are ordinarily viewed at a bright location, and soft copies are often viewed by setting the ambient light to be slightly dark. Further, soft copies are of the light emission colors, and therefore the color appearances given by the soft copies differ from the color appearances given by the reflecting materials, such as the hard copies.
As for how an image is to be transformed by considering the viewing conditions (such as the correlated color temperature of the white color, the luminance, and the ambient conditions) such that the same color appearances may be obtained when the image is viewed under certain conditions and when the image is viewed under different conditions, it has been proposed to employ transformation methods which are ordinarily referred to as the chromatic adaptation transformation or color appearance models. The transformation methods are described in, for example, Japanese Patent Publication Nos. 7(1995)-86814 and 7(1995)-86815; "Color Research and Application," Volume 19, Number 1, 1994, R. W. G. Hunt; "Color Research and Application," Vol. 20, No. 3, 1995, N. Nayatani; and "Color Research and Application," Vol. 16, No. 4, 1991, M. D. Fairchild.
However, the proposed models are based upon experimental results derived by using a single stimulus, such as a color patch or a color chip, and are not based upon image transformation between different media for processing a soft copy and a hard copy and for processing pictorial images. Further, the methods described in the Hunt's and Nayatani's literatures require very complicated calculations and very large amounts of calculations and are therefore not always appropriate for the applications of image transformation.
In cases where the aforesaid chromatic adaptation models or the color vision models are used, when a hard copy or a soft copy is viewed, it is basically necessary for the human visual system to be completely adapted to the white colors of the hard copy and the soft copy. However, when a soft copy image is compared with a hard copy image, the substantially adapting illuminant is not necessarily be the white color itself of the CRT display device. For example, the white color of a CRT display device having been set at a correlated color temperature of 5000K is ordinarily perceived as white slightly tinted with yellow, and the white color of a CRT display device having been set at a correlated color temperature of 9300K is ordinarily perceived as white slightly tinted with blue. The phenomenon indicates that the human visual system is not completely adapted to the white color of the CRT display device. The phenomenon is known as incomplete chromatic adaptation.
With respect to the incomplete chromatic adaptation, a suggestion is made in, for example, "Appearance Match between Soft Copy and Hard Copy under Mixed Chromatic Adaptation" by N. Katoh, Proceedings of the IS&T/SID 1995 Color Imaging Conference: Color Science, Systems and Applications. In the literature, it is suggested that, in order for the color appearances to be matched when an image on a CRT monitor is viewed under ambient lighting, good prediction can be made with respect to an adapting illuminance by carrying out a calculation in accordance with an RLAB color vision model (the Fairchild's method described above), in which a 6:4 internally dividing point between the chromaticity of the CRT monitor's white point expressed with the physiological primary colors and the point of the ambient lighting's white point expressed with the physiological primary colors is taken as the adapting white point. However, in the RLAB model, the effects of offsetting the tint of the illuminant (i.e., the effects of reducing the tint toward the equi-energy white color) are taken into consideration. Specifically, the internally dividing point between the point, which results from the subtraction of the tint of the illuminant, and the white color of the ambient lighting is taken as the adapting white point, and therefore problems occur. Also, it is known that, for example, as the surround becomes dark, a dark color is perceived as being comparatively bright, and the apparent contrast or the apparent color vividness becomes low. Therefore, a need exists for a simple image transforming method, with which the compensation for the effects occurring from the brightness of the surround can be made together with the aforesaid compensation for the chromatic adaptation.